Development of
- Improved catalysts to be employed within existing production units for existing reactors
- Improved catalysts for existing reactors using new procedures calling for new equipment
- Integration of catalyst and reactor
Heat transfer and mass transport
- Integration of catalytic reaction and separation of reactants or reaction products
Catalytic distillation as an example : performing a catalytic reaction within a distillation column ......
2. Objectives of
the Design of Solid Catalysts
Development of
Improved catalysts to be employed within
existing production units for existing reactors
Improved catalysts for existing reactors using
new procedures calling for new equipment
Integration of catalyst and reactor
Heat transfer and mass transport
Integration of catalytic reaction and separation of
reactants or reaction products
Catalytic distillation as an example :
performing a catalytic reaction within a
distillation column
3. Objectives of
the Design of Solid Catalysts
Most smooth penetration into market for
• Improved catalysts to be employed within existing
production units for existing reactors
• Relatively small rise in conversion and/or selectivity
leads to large increase in profit at low costs of
investment
Other possibilities are calling for higher costs of
investment and are thus more difficult to get accepted
• Smaller units more easy access to the market; new
concepts to be introduced first in small-scale units
4. Development of Solid Catalysts
Mechanical strength
Most important for technical applications
Size of catalyst bodies
Determining pressure drop
Active surface area
Sufficiently large active surface area per unit volume of
catalyst
Active surface area stable at temperatures of
pretreatment and catalytic reactions
Desired structure and chemical composition
5. Development of Solid Catalysts
Transport of material and thermal energy to
and from the active sites
• Solid catalysts usually highly porous and thus
thermally isolating materials
• Length of pores more important than diameter of
pores (Thiele’s modulus)
Tri-lobs, quadri-lobs, rings
Liquid-phase catalysts completely different
constraints than gas-phase catalysts
• Development of gas-phase catalysts much more
advanced
6. Design of Solid Catalysts
Catalytic activity and selectivity
Structure and chemical composition of active surface
Extent of active surface area
Transport properties
To catalyst bodies
Size and shape of catalyst bodies; pressure drop
Within catalyst bodies
Size of catalyst bodies
Pores size distribution
7. Supported Solid Catalysts
Usual separation of functions
Support
Size and mechanical strength of catalyst
bodies
Porous structure
Active component
Structure and chemical composition of
catalytically active surface
8. Supported Solid Catalysts
However, sometimes catalytic function of support
also involved : Bi-functional catalysts
• Acid function of support with precious metal
function in catalytic reforming catalysts
Support promoting dissociation of carbon monoxide
in some Fischer-Tropsch catalysts
• Different selectivity of titania-supported cobalt
catalysts
Reduced titanium ions at the periphery of the
supported metal particles take up oxygen of
carbon monoxide and thus promote dissociation
9. Unsupported Catalysts
Reasons to employ unsupported catalysts :
Active species providing sufficiently large and
thermo stable surface area as well as suitable pore
structure
Catalytically active species capable of providing
sufficiently strong porous bodies
Reaction of suitable supports with required
promoters, which prevents promoters to be
effective
10. Unsupported Catalysts
Examples :
• Pt or Pd gauze for the oxidation of ammonia to
nitrogen oxide in the production of nitric acid and
Pt gauze in Andrussow’s process for the
production of HCN from methane and ammonia
• Silica-alumina cracking catalyst
• Raney metal catalysts
• High-temperature carbon monoxide shift
conversion catalyst : iron oxide-chromium oxide
11. Unsupported Catalysts
Examples :
• Ethylbenzene dehydrogenation catalyst (iron
oxide promoted with potassium oxide)
Potassium oxide promoter reacting with the
usual alumina and silica support
• V-P-O catalyst for the selective oxidation of n-
butane to Maleic anhydride
Alumina support reacts with phosphoric acid
and disturbs vanadium/phosphorous ratio;
vanadium difficult to apply to silica support
12. Catalyst Preparation :
Science or Art
First example :
Preparation of VPO catalyst for oxidation of n-
butane to Maleic anhydride
Procedure (1) Centi et al.
Reduction of 6.7 g of V2O5 for 16 hours in
80 ml of HCl at 100oC
Addition of 9.3 g 85% H3PO4 and refluxing the
solution for 1 hour
Evaporate to dryness
Dry resulting green viscous mass in nitrogen
flow for 10 hours at 125oC
13. Catalyst Preparation :
Art or Science
Procedure (2) Katsumoto et al.
Reduction of 15 g V2O5 at 120oC in 60 ml
1:1 (v/v) i-butanol/cyclohexanol mixture
Cooling to room temperature and addition of
21 g of o-H3PO4 mixed with 30 ml butanol
Refluxing for 6 hours leads to blue-green
suspension
Filtering of suspension and drying in nitrogen
flow for 12 hours at 125oC
14. Catalyst Preparation :
Art or Science
Reduction of vanadium to mixture of vanadium(IV)
and vanadium(V) by either inorganic (HCl) or organic
reducing agent
Catalysts produced in either way call for being at
least for 24 h on stream to exhibit a reasonable
selectivity and activity
15. Solid Catalysts
Usually supported catalysts in view of better control of
properties of catalysts
Surface area and loading of the support with the active
component as well as the distribution of the active
component over the surface of the support determining
• Extent of catalytically active surface area per unit
volume
• Thermo stability together with the interaction of the
active component with the surface of the support
17. Components of Solid Catalysts
Support
Shape and size of catalyst bodies
Porous structure
Mechanical strength
Surface area
Active component
Size and number (loading) of supported active moieties
Distribution over surface of support
Interaction with support
18. Types of Supported Catalysts
Catalysts containing base metals, base metal oxides or
base metal sulfides
Active surface area per unit volume decisive
Limiting size of reactor
Usually high loadings of support with active
component(s)
Loadings of 20 to 50 wt.% usual
Catalysts containing precious metals
Active surface area per unit weight of precious
metal decisive
Low loadings of active component(s)
Less than 1 wt.% usual, sometimes up to about 5
wt.%
19. Production of Finely Divided
Material
Condensation of molecularly dispersed species
Selective removal of some component
20. Preparation Procedures of
Supported Catalysts
Application of active precursor to separately produced
support
Application of active precursor into pre-shaped
support bodies
Application on powdered support and subsequent
shaping
Selective removal of one or more constituents from
essentially non-porous precursor of support and active
component
Examples Raney metals; ammonia synthesis
catalyst; methanol and low-temperature carbon
monoxide shift catalyst based on copper/zinc oxide
21. Preparation of Catalysts by
Selective Removal
Resulting in
powder, e.g., Raney nickel
powder to be processed to bodies, e.g.,
methanol synthesis catalyst
porous solid bodies, e.g., ammonia synthesis
catalyst
Non-porous
Precursof
Powdered catalyst
Porous catalyst body
Shaped
catalyst body
22. Catalyst Preparation: Art or
Science
Second example :
Ammonia synthesis catalyst
Trial and error
Iron ore
About 97% Fe3O4 2% Al2O3 1% K2O
Double-promoted iron catalyst
Alumina structural promoter
Potassium required to maintain activity
at higher pressures
23. Catalyst Preparation: Science
or Art
Selective removal of oxygen
Minimum amount of Al2O3 to transport water rapidly
out of porous catalyst bodies
Al2O3 effectively prevents sintering
Role of potassium still debated
Presumably potassium oxide promoting
desorption of ammonia, which is required at
elevated pressures
24. Effect of Structure of Active Surface
Different surface structures
Different activity
per unit surface area
Effect of size of active particles
Large active particles
mainly atomically flat
surfaces
Small active particles
penetration of atoms
into surface layer
27. Generally Employed Supports
By far the most preferred commercial support : Alumina
due to elevated bulk density
• Usually g-alumina, surface area from about 300 to 100
m2/g; most preferable needles from boehmite (AlOOH),
less preferable from gibbsite or bayerite (Al(OH)3)
• a-alumina is support with relatively inert surface and
surface area of usually less than 1 m2/g and
exceptionally 10 m2/g
When alumina cannot be employed, silica is the second
best
28. Support for Precious Metals
Precious metals used in liquid-phase processes :
Activated carbon attractive support
• Carbon is not attacked by acids and alkaline liquids
• Carbon bodies of about 50 mm therefore often used
suspended in liquids
• To reclaim the precious metal a carbon support can
be removed by simple combustion
29. Activated Carbon Supports
Since activated carbon is produced from peat or wood,
it is a natural product and therefore difficult to
reproduce accurately
Mechanical strength of carbon supports is often
problematic
(Carbon supports cannot be calcined in air)
Apparent surface area of activated carbon about 1200
m2/g
• Activated carbon contains many micropores
• Besides very small particles also stacking of
graphite layers present
30. Commercial
Pre-Shaped Support Bodies
Main deficiency of alumina and silica supports : not
compatible with alkaline promoter species
Silica is volatile with steam at high pressure and/or high
temperatures
Other supports, such as, zirconia or titania more difficult to
process to mechanically strong bodies of an elevated
surface area; supports are much more expensive
• Zirconia and titania compatible with alkaline materials
• Alternative supports much more expensive
Important producer of alternative supports : HAISO
31. Preparation Procedures of
Supported Catalysts
Application of active precursor onto
separately produced support
• Application of active precursor into pre-
shaped support bodies
• Application on powdered support and
subsequent shaping
Employing commercial pre-shaped support
bodies most obvious
• Wide range of different shapes and sizes of
alumina and silica available
32. Preparation of Supported
Precious Metal Catalysts
Most obvious procedure
Pore-volume impregnation and drying of pre-shaped support bodies
Actually adsorption of active precursor on surface of support
Alumina impregnation with acid, negatively charged precursors
Silica impregnation with positively charged ammonia complexes
With alumina neutralization of acid components of
impregnating liquid often important
No risk of loss of precious metal with, e.g., waste water
Selection of size, shape, pore structure, and mechanical strength of
support bodies viable from large range of commercial support
bodies
34. Preparation of Supported
Precious Metal Catalysts
Precious metal precursor often adsorbing on surface
support
At the usual low loadings of precious metals
adsorption brings about inhomogeneous distribution
of the precious metal over the support bodies
Chromatographic effect
35. Distribution of
Active Precious Metal Particles
Uniform distribution
of precious metal particles
Usually desired when
transport limitations are
not expected
Egg-shell distribution
of precious metal particles
Resulting from adsorption from
the impregnating liquid
Only desired with transport limitations
36. Preparation of Supported
Precious Metal Catalysts
Alternative industrial procedure to arrive at egg-shell
distribution of precious metal particles :
Spraying of solution of dissolved precious metal
precursor onto agitated volume of support bodies
To limit penetration of solution of active precursor into
pores of support, support is often pre-heated
Egg-shell distribution of active precious metal particles
on support bodies often desired with catalysts intended
for liquid-phase processes
37. Preparation of Supported
Precious Metal Catalysts
Establishment of an egg-shell distribution of precious
metal particles on alumina support bodies
Fill pore volume of pre-shaped support bodies
completely with water
Pass acid solution of precious metal along water-filled
support bodies
• Neutralization of acid solution at external surface of
alumina support and consequent deposition of
palladium compound
• Slow transport of dissolved precious metal species
through water present within pore system of support
38. Preparation of Supported
Precious Metal Catalysts
Soaking of alumina support in solution of precious
metal or recirculation of solution of precious
metal for long periods of time leads to uniform
distribution and high loading
39. Preparation of Supported
Precious Metal Catalysts
Uniform distribution of precious metal(s) can be
achieved more readily by employing less strongly
adsorbing species
• With alumina supports change H2PtCl6 for K2PtCl6
Generation of adsorbing Al+ sites by reaction of
proton with surface OH- groups of alumina
Reportedly PtCl6
2- generating protons by exchange
with water and dissociation of water, which
proceeds slowly
•
PtCl6
2- + H2O = PtCl5(H2O)- + Cl-
PtCl5(H2O)- + H2O = PtCl5(OH)- + H3O+
40. Preparation of Supported
Precious Metal Catalysts
Control of location of active component within
support bodies
Competitive adsorption with dibasic organic acids,
e.g., oxalic acid, tartaric acid, citric acid or
aromatic acids with hydroxyl group besides
carboxyl group, as, e.g., salicylic acid
Homogeneously applied Egg-shell Egg-white
41. Preparation of Supported
Precious Metal Catalysts
Adsorption of precious metal on activated carbon
• Freshly produced activated carbon hydrophobic
• Storage without exposure to atmospheric air
maintains hydrophobicity
• Usual activated carbon hydrophilic due to surface
oxidation leading to carboxylic acid groups
• Oxidation, e.g., by hydrogen peroxide, nitric acid
(cautious for explosions) or ozone can increase
number of carboxylic acid sites and thus raises
hydrophilicity
42. Adsorption of Precious
Metals on Activated Carbon
Limited adsorption of positively charged complexes of
precious metals, such as, ammonia complexes
More extensive loading from strongly acid solutions of
precious metals
• Reason not completely clear
Adsorption on positively charged carboxyl acids
due to uptake of additional proton
More likely good wetting of carbon by acidic
solution and deposition by evaporation of liquid as
species badly crystallizing from acid film on carbon
surface
43. Preparation of Supported
Precious Metal Catalysts
With pre-shaped silica supports preferably impregnation
with positively charged precious metal complexes
• Ammonia complexes attractive
• Organic nitrogen complexes may lead to reduction of
precious metals at slightly elevated temperatures
At pH levels above about 2 silica increasingly negatively
charged due to dissociation of surface hydroxyl groups;
only at pH levels above about 6 sufficient reactivity of silica
At more elevated pH levels dissolution of finely divided
silica to be considered
44. Impregnation of Pre-Shaped
Support Bodies and Drying
Impregnating with active precursor solution not (strongly)
adsorbing on surface of support
Most rapid procedure to arrive at industrial catalysts
Selection of appropriate support from wide range of
commercial supports
Pore volume and solubility of active precursor determine
maximum loading
Difficult to achieve uniformly distributed active
component(s)
Often concentration of active component at external
edge of support bodies
45. Production of Supported
Catalysts by Impregnation and
Drying
Impregnation with solutions of species not
strongly adsorbing on surface of support
Higher loadings can be achieved than with
adsorbing species
Difficult to achieve :
Uniform distribution within support body
46. Production of Supported
Catalysts by Impregnation and
Drying
Impregnation and drying of pre-shaped support bodies
Evacuation of pre-shaped support bodies
• Laboratory-scale catalyst preparation : employ vapor
• Addition of volume of impregnating solution equal to
pore-volume to evacuated support
Dry (under vacuum) at room temperature and
subsequently at increasingly higher temperatures up to
about 120 to 150oC
Subsequent calcination at about 350 to 500oC
47. Production of Supported
Catalysts by Impregnation and
Drying
Impregnation of porous support bodies filled with air
may lead to fracture of bodies when the amount of
liquid is larger than the pore volume
• Experiments on sol-gel silica spheres produced by
GBHE upon immersion in water
With pore volume impregnation some volume
elements may not be penetrated by impregnating
liquid
• Volume elements not containing active components
48. Impregnation and Drying of
Pre-Shaped Support Bodies
Often Active Precursor selectively Deposited on
External Edge of Support Bodies
With Supports of Wide Pores to be Expected
With all Hydrophilic Supports Migration of Liquid
to External Edge
49. Support Bodies having Wide
Pores
Incipient wetness impregnation and drying
Deposition of active precursor at external edge
Result : Eggshell catalyst
50. Shape of Drop dried on Glass
Drop of solution of copper(II) nitrate dried on microscope glass slide
Note preferential build up of crystallites at the rim of the drop
51. Shape of Drop dried on Glass
Drop of solution of copper(II) citrate dried on microscope glass slide
Note absence of large crystallites
52. Preparation of Supported Catalysts
by Impregnation and Drying
Also with supports having fairly narrow pores
deposition of active precursor at external edge of bodies
Evaporation of liquid at external edge and transport of
liquid to external edge
Achieve uniform distribution throughout support bodies
by using solutions of badly crystallizing active
precursors the viscosity of which raises when the
solvent is removed by volatilization
Citric acid complexes
EDTA complexes
Addition of, e.g., sugar (prevent explosive
decomposition)
53. Preparation of Supported Catalysts
by Impregnation and Drying
Upon suitable impregnation pores of support are
uniformly filled with solution of active precursor,
provided no substantial adsorption on the surface of the
support proceeds
Accumulation of active species at external edge of
support bodies is established during drying
• During the main part of the drying process the
evaporation of solvent takes place exclusively at the
external edge of the support bodies
• Transport of water vapor within porous structure
proceeds too slowly to lead to significant gradients in
partial pressure
54. Preparation of Supported Catalysts
by Impregnation and Drying
Hydrostatic pressure of liquid within porous system of
support bodies determined by capillary forces
ΔP = 2γ/r
• ΔP pressure difference between air pressure outside
pore system and hydrostatic pressure within system
having pores of radius r at the external edge
• g surface energy of the liquid/gas boundary;
water 72.88 dyne/cm 20oC 71.40 dyne/cm 30oC
• Since r the radius of curvature of the meniscus is
negative, the hydrostatic pressure in the liquid is
lower than the air pressure
55. Stages during Evaporation of
Solvent
Initial stage
Second stage
Formation of
menisci at the
liquid-gas interface
Radii of menisci
between different
particles equal
Four stages during the evaporation of the solvent of an impregnated
support body
Third stage
Haines jump
by emptying
of volume V1
Fourth stage
Transport through
adsorbed liquid
film to external
surface within
funicular region
57. Evaporation of Solvent from
Impregnated Support Bodies
Transport of liquid also as a liquid film over the
surface of the support much more rapid than transport
of vapor through emptied pores of support
• Evident from the fact that the rate of evaporation is
constant during a large fraction of the drying
process
• In the last stage of the drying process the liquid film
has disappeared from a significant fraction of the
volume of the support body, this is stage is known
as the pendular state
59. Formation of Bubbles within Liquid
present in Impregnated Support Bodies
Formation of bubbles of
vapor of solvent during
evaporation of solvent
from impregnated support
bodies
Bubbles can only arise within
pores of a diameter larger
than the diameter of the
necks between the elementary
particles at the external edge
60. Rate of Drying of Porous Bodies
Impregnated with Water
GBHE A2ST a-alumina ring extrudates 0.27m2/g, 0.40 ml/g, void fraction 0.62
GBHE A2ST silica spheres 70 m2/g, 0.85 ml/g, void fraction 0.66
GBHE A2ST
61. Rate of Drying of Porous Bodies
Impregnated with Water
GBHE A2ST silica spheres 70 m2/g, 0.85 ml/g, void fraction 0.66
GBHE A2ST a-alumina cyl. Tablets 1.0 m2/g, 0.20 ml/g, void fraction 0.45
GBHE A2ST
62. Impregnation and Drying of
Support Bodies
Apparently transport of liquid either through filled pores or
as a liquid film on the surface of the support to the external
edge of the support bodies
With the liquid the dissolved species is migrating, which
leads to deposition of the active precursor at the external
edge of the support body
Important parameters : viscosity of the liquid, especially as
a function of the concentration, and the interaction of the
liquid with the surface of the support
63. Impregnation and Drying of
Support Bodies
Impregnation with different dissolved iron(III)
species
Evaluation of the distribution obtained after drying
and calcination by X-ray diffraction
• Finally divided material evident from absence of
sharp X-ray diffraction profiles
64. Impregnation of Silica with
Different Iron Precursors
Support : Silica extrudates 2.1 mm diameter
Prepared from Ox 50 Degussa
Surface area 44 m2/g; pore volume 0.8 ml/g
void fraction 0.65
Fe EDTA pH 8.5
Fe EDTA pH 10.1
Fe gluconate
NH4 Fe citrate
Fe(NH4)2(SO4)2.6H2O
Fe2(SO4)3.5H2O)
FeCl3.6H2O
Fe(NO3)3.9H20
65. Impregnation of Silica with
Different Iron Precursors
Simple, well soluble salts of iron(III) lead to deposition of relatively
large crystallites, mainly at the external edge of the support bodies
Complexes of iron with organic ligands or the presence of
dissolved organic species containing hydroxyl groups or other
hydrophilic groups substantially improves the distribution of the
iron species over the support
Interesting is the effect of the pH value of the impregnating liquid
with the iron(III) EDTA complexes
66. Impregnation and Drying of
Support Bodies
Employing a liquid the viscosity of which rises when
the solvent is evaporating suppresses motion of the
liquid as an adsorbed film to the external edge of the
support bodies
Interesting to establish the viscosity of the
impregnating liquid as a function of the
concentration taking into account that evaporation
of the solvent leads to an increase in concentration
68. Impregnation and Drying of
Support Bodies
Effect of interaction of the species of the active
precursor deposited from the solution with the surface of
the support
• An effect of interaction with the support indicated by
the effect of the pH of the impregnating solution with
the iron(III) EDTA complexes
Experiments with silicon wafers covered by a thin silica
layer upon exposure to atmospheric air
• Application of thin layer of solution by spin coating
• Evaluation of the deposition by AFM (Atomic Force
Microscopy)
69. Deposition of Copper Oxide
on Silicon Wafers
Deposition of clusters of copper(II) oxide particles from a solution of
Cu(NO3)2 in cyclohexane on “natural” oxide layer present on silicon
wafers
When some small particles
have been deposited, the
remaining solution
is taken up within
the pores in between the
particles due to capillary
forces
Note high magnification
70. Deposition of Copper Oxide
on Silicon Wafers
Silica layer on silicon wafers hydrophobic, treatment with ammonia and H2O2
required to produce a hydrophilic silica layer.
On the hydrophilic
silica surface, much more
interaction with the
solution, which leads to
deposition of well
distributed small copper
oxide particles
71. Deposition of Iron Oxide on
Silica Extrudates
Confirming effect of interaction with surface of support
by experiments with silica extrudates
• Impregnation with a solution of Fe(NO3)3 and
subsequent drying
Calcined at 750oC Fresh Treated at 100oC
with NH3/H2O2/H2O)
72. Impregnation and Drying of
Support Bodies
Interaction of the species to be deposited with the
surface of the support certainly important as evident
from the effect of pretreatment of silica surfaces
Effect of organic species only rise in viscosity or is
organic species also enhancing the interaction with the
surface of the support ?
73. Elution Experiments with Different
Iron Species adsorbed on Silicagel
Solutions of different iron compounds brought on silica
gel column
Subsequently eluted with water of the same pH as the
initial iron solution
Elution volumes reflecting interaction with silica surface
Precursor salt Elution volume (ml)
(NH4)2Fe(SO4)2 6.2
FeCl3 6.3
Fe(NO3)3 7.7
NH4 Fe citrate 5.6
74. Elution Experiments with Different
Iron Species adsorbed on Silica gel
Apparently interaction of dissolved species behaving
completely differently during drying after impregnation
not significantly different
Spin-coating experiments with silicon wafers
pretreated with ammonia and hydrogen peroxide
75. Deposition of Iron Oxide on Silicon
Wafers by Spincoating
Before investigation in the AFM the wafers have been calcined
(NH4)2Fe(II)(SO4)2 FeCl3
Fe(NO3)2
NH4 Fe citrate
Wafer surfaces pre-treated with NH4/H2)2/H20
76. Deposition of Iron Oxide on
Silicon Wafers by Spin coating
Apparently at room temperature interaction of support
(silica) surface with dissolved species containing organic
molecules not substantially different
At more elevated temperatures interaction much more
stronger leading to tenaciously adhering film to surface of
the support
Due to elevated viscosity growth of crystal nuclei
impeded within the film layer
Other organic molecules containing hydroxy groups, such
as, sugar or HEC exhibit same effect
77. Impregnation of a-Alumina with
Different K3Fe(CN)6 Precursors
No HEC 1 wt.% HEC 2 wt.% HEC
HEC = hydroxy ethyl cellulose
79. Conclusions about
Impregnation and Drying
Impregnation and drying of pre-shaped support bodies
excellent and rapid procedure to produce supported
catalysts
• No waste water; no loss of active species
• Scale up can be performed readily
At low loadings of active species adsorption on the
surface of the support can allow one to control the range
within the support body where the active precursor will
be deposited
80. Conclusions about
Impregnation and Drying
To achieve higher loadings adsorption of active
precursors on the surface of the support can not
effectively be employed
Bad distributions of the active precursors within the
support bodies is due to migration of liquid elements
during evaporation of the solvent at the external edge
of the support bodies
81. Conclusions about
Impregnation and Drying
Impregnation with solutions the viscosity of which
increases during evaporation of the liquid is very
effective in establishing a uniform distribution of the
active species throughout the support bodies
The agent raising the viscosity generally also increases
the interaction with the surface of the support, but as
required only at elevated temperatures when the
solvent has largely evaporated